1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus in which the operation of a control circuit is started and stopped in accordance with an input AC voltage.
2. Description of the Related Art
Power supply apparatuses that employ switching elements are widely prevalent due to having a high conversion efficiency. Japanese Patent Laid-Open No. 2007-006614 proposes a current resonance power supply in which the voltage between both ends of one switching element is detected, and another switching element is switched on when the detected voltage has risen to a predetermined voltage or higher, thus preventing a short circuit caused by resonance deviation when an electrical overload occurs.
According to Japanese Patent Laid-Open No. 2007-006614, a voltage detection circuit detects the voltage at both ends of one switching element and outputs the detected voltage to a control circuit. Generally, only a low-level voltage can be applied to the input terminal of a control circuit, therefore the voltage detection circuit needs a voltage-dividing circuit for dividing a relatively high voltage such as a commercial voltage. Since this voltage-dividing circuit consumes power even when the power supply apparatus shifts to an energy-saving operation mode (a low load mode), power consumption tends to rise.
Incidentally, a control IC controls the operation of the switching elements included in a power supply apparatus, and the control IC includes an enable terminal. The control IC starts operating when a voltage Vsns applied to the enable terminal rises to an operation start voltage Vstart or higher. However, there are cases where a decrease occurs in an input AC voltage Vin that is supplied from a commercial power supply to the power supply apparatus after the control IC has started operating. If the input AC voltage Vin falls to an operation stop voltage Vstop or lower, the current flowing to the primary side becomes excessive in an attempt to maintain the voltage on the secondary side. When the current on the primary side becomes excessive, elements become damaged and the conversion efficiency decreases. In view of this, the control IC is designed so as to stop operating when the input AC voltage Vin falls to the operation stop voltage Vstop or lower.
FIG. 6A shows the relationship between the voltage Vsns at the enable terminal of the control IC and the input AC voltage Vin in an ideal state. In this example, the control IC starts operating when the input AC voltage Vin rises to 80 V or higher, and the control IC stops operating when the input AC voltage Vin falls to 60 V or lower. The control IC starts operating when the voltage Vsns at the enable terminal rises to the operation start voltage Vstart or higher, which is proportional to the input AC voltage Vin of 80 V, and the control IC stops operating when the voltage Vsns falls to the operation stop voltage Vstop or lower, which is proportional to the input AC voltage Vin of 60 V. In this way, the operation start voltage Vstart needs to correspond to 80 V, and the operation stop voltage Vstop needs to correspond to 60 V. However, the operation start voltage Vstart and the operation stop voltage Vstop vary under various circumstances.
FIG. 6B shows the case where the operation start voltage Vstart has become too high. In this example, the operation start voltage Vstart has risen to a voltage that corresponds to the input AC voltage Vin of 100 V, and therefore the control IC cannot start even if the input AC voltage Vin has risen to 80 V or higher.
FIG. 6C shows the case where the operation stop voltage Vstop has become too low. In this example, the operation stop voltage Vstop has decreased to a voltage that corresponds to the input AC voltage Vin of 45 V, and therefore the control IC fails to stop even if the input AC voltage Vin has fallen to 60 V or lower.